The treatment of solid coal to obtain liquids therefrom has been extensively developed and a variety of coal liquefaction processes have been reported. These processes include solvent refining, direct hydrogenation of coal, catalytic or noncatalytic hydrogenation in the presence of a hydrogen donor solvent and catalytic or noncatalytic hydrogenation in the presence of a vehicle or nondonor material. Generally, these processes can be characterized as involving substantial chemical conversion of the coal as opposed to solvent extraction with solvents such as pyridine, benzene, tetrahydrofuran, etc. In coal liquefaction, conversion of the coal is normally effected at elevated pressures and elevated temperatures, e.g., above 150.degree. C., preferably above about 250.degree. C., while solvent extraction is normally effected at room temperature or temperatures up to the boiling point of the solvent employed.
Coal liquefaction processes wherein a coking zone is incorporated to treat the heavier or bottoms portion of the liquids from the liquefaction reactor have been extensively reported. For example, U.S. Pat. No. 3,617,513 discloses a coal liquefaction process wherein a fluid coker is employed to enhance liquid product yield. The bottoms stream comprising depleted donor solvent, heavy liquids and unconverted or unreacted coal is sent to the fluid coking zone from which light gases, liquids and coke are recovered. In the coking zone, heavy liquids, e.g., 1000.degree. F.+ are removed from a distillation tower located immediately above the coking zone for further conversion. Since coke precursors include high boiling condensed ring aromatic molecules, the coke precursors will be recycled to the coking zone. It has now been discovered, however, that by recycling these heavy materials to the liquefaction zone where the materials are treated under hydrogenation conditions, the yield and quality of the liquid products of the process are enhanced considerably.